The present invention relates generally to coded orthogonal frequency-division multiplexing (OFDM) systems, and more particularly to reducing the peak-to-average power ratio (PAPR) in coded OFDM systems.
Orthogonal frequency-division multiplexing (OFDM) has traditionally been robust against multipath fading channels and may be used for effective high-speed wireless data transmission.
One drawback of OFDM systems, however, is that an OFDM signal typically exhibits a high peak-to-average power ratio (PAPR). Such a high PAPR occurs when symbols that create the OFDM signal multiplied with the Inverse Discrete Fourier Transform (IDFT) add constructively.
A high PAPR often necessitates the use of a linear amplifier with large dynamic range (the range in which the amplifier has linear amplifying property). A linear amplifier having a large dynamic range is often difficult to design. An amplifier with nonlinear characteristics (i.e., outside of the linear operation range), however, can cause undesired distortion of the in-band and out-of-band signals.
A number of approaches have been proposed to suppress the PAPRs in OFDM systems. These approaches may be grouped into different categories or techniques. One technique to suppress the PAPRs in OFDM systems is by using block coding (i.e., to transmit codewords having low PAPR). Such coding techniques typically provide acceptable PAPR reduction and coding gain. A problem associated with the coding approach is that, for an OFDM system with a large number of subcarriers, either the system encounters design difficulties or the coding rate becomes prohibitively low.
The second type of approach is through clipping and filtering of OFDM signals. Clipping can reduce PAPR, but may introduce in-band clipping noise and filtering. Filtering is employed to remove side-lobes generated by clipping, but filtering may also generate additional PAPR.
The third type of approach is phase rotation including selective mapping (SLM) and partial transmit sequence (PTS). A PAPR reduction scheme may use advanced codes, such as turbo codes, low-density parity-check (LDPC) codes, or repeat accumulate (RA) codes to achieve SLM. These codes not only offer high error correction performance, but a random interleaver in an encoder also provides different random coded sequences for SLM using several label bits before encoding. The sequence is a sequence of coded bits after the encoding, i.e., a codeword. With different settings of the label bits, different codewords are obtained (i.e., different sequences of the coded bits). However, a disadvantage of the conventional SLM or PTS technique is that one or more iterative gradient algorithms may need to be applied to reduce the complexity of searching for the optimal sequence over candidate sequences.
There is no such gradient methods for complexity reduction in a coded scrambling method (e.g., described above using the different label-bits to obtain different coded sequences). Instead, a selector usually has to exhaustively travel through all of the sequences obtained from the different combinations of label bits.
Therefore, there remains a need to more effectively reduce the PAPR of an OFDM system.